Contrary to popular wisdom, eating at McDonald’s isn’t exactly cheap, costing some $28 for a family of four. Which might help explain the results of a recent study from the University of California, Davis, which found that people’s visits to fast-food joints increased along with their incomes, and that poor people were spending fewer dollars on fast food than lower-middle and middle-income Americans.

The authors said their study suggests that the availability of fast food isn’t the only driver of obesity in poor groups. “There is a correlation between obesity and lower income, but it cannot be solely attributed to restaurant choice,” said J. Paul Leigh, professor of public health sciences at U.C. Davis and senior author of the study, in a statement. “Fast-food dining is most popular among the middle class, who are less likely to be obese.”

Leigh and colleague DaeHwan Kim analyzed 1994-96 data from the Continuing Survey of Food Intakes by Individuals and the accompanying Diet and Health Knowledge Survey. The data included responses from 5,000 Americans who were asked about restaurant dining habits, income, race, gender, age and education.

The researchers found that people visited fast-food restaurants more often as their household income increased — at least up to a point. Fast-food visits rose along with annual income up to $60,000; beyond that, visits started to drop back down, replaced by full-service, sit-down dining at slightly higher prices.

Based on the data, the researchers described the typical fast-food consumer as a lower-middle income head of household, who is budget-conscious and harried and likes the convenience and low price of fast food, compared with other restaurants. Poor people, by contrast, can’t easily afford fast-food “value meals,” and the poorest, who may rely on the FNS Supplemental Nutrition Assistance Program, or SNAP, certainly can’t use food stamps at McDonald’s.

For many years, the connection between poverty and obesity has been linked in part to the abundance of fast food in low-income areas. While the current study challenges the notion that poorer people eat more fast food than those who are better off, it doesn’t absolve fast-food restaurants completely.

The study didn’t take into consideration what people ate outside of restaurants, for example, and it’s well established that low-income neighborhoods tend to be “food deserts” — where fresh, whole foods are scarce and where the bulk of available food is the high-fat, high-sugar stock of convenience stores. That type of environment is thought to contribute to unhealthy eating and weight gain.

“It would be a big mistake to look at the results of this report and say the environments people live in don’t matter, because they do,” Micah Weinberg, a senior policy adviser at the Bay Area Council who works on public health issues, told The Sacramento Bee.

The study’s authors agreed that fast food certainly isn’t helping the national waistline. “I’m not a big fan of fast food,” Leigh told the Bee. “I’m sure that fast food in general has a big effect on obesity. This research does not contradict that.”

The study will be published in Population Health Management in December